The present invention relates to a process for producing sorbitol and gluconic acid or gluconate from aqueous mixtures of fructose and glucose using permeabilized cells of Zymomonas mobilis in gel-immobilized form.
The production of sorbitol and gluconic acid starting from mixtures of glucose and fructose by means of Zymomonas mobilis is known. U.S. Pat. No. 4,755,467 describes the use of permeabilized cells for this process, and these methods are intended, where appropriate, to be employed in immobilized culture. Permeabilization of the cells was carried out by exposure to toluene, however, no disclosure of specific agents for the immobilization of such cells was disclosed.
A particularly expedient method of permeabilizing Zymomonas mobilis cells for the production of sorbitol and gluconic acid is described in German Patent Document 38 41 702 (=U.S. Ser. No. 448,334) now U.S. Pat. No. 5,017,485, in which a freezing technique is described. This method produces particularly useful permeabilized cells. In addition, a process for producing sorbitol and gluconic acid using Zymomonas mobilis cells permeabilized with a cationic surfactant is described in German Patent Application 39 36 757.6, which corresponds to U.S. Ser. No. 606,821 now U.S. Pat. No. 5,102,795 this application, the use of immobilized cells is also recommended, but no specific support materials are disclosed.
The use of inorganic support materials such as, in particular, sintered glass or ceramic is widespread in biotechnological processes. M. T. Bomer proposes, in ZFL 6/85, pages 424-425, the use of agar gel or alginates as support materials for the immobilization of microorganisms, such as E. coli. In such applications, agar gel is preferred because of its low toxicity.
D. H. Chun et al. (Appl. Microbiol. Biotechnol. 29: 19-24 (1988)) describe the production of sorbitol and gluconic acid using toluene-permeabilized cells of Zymomonas mobilis immobilized in sodium alginate/Celite.RTM. mixtures. These immobilizates are rigidified with a calcium chloride solution.
S. L. Paterson (Biocatalvsis vol. 1, pages 217-229 (1989)) describes the production of sorbitol and gluconic acid in a hollow-fiber membrane reactor in which toluene-treated cells of Zymomonas mobilis are "immobilized."
In the processes described above, the efficiency of production of sorbitol and gluconic acid is improved by the use of permeabilized cells because no nutrients are needed for biomass production. There is, however, a relatively rapid decrease in the activity of the enzymes important to the conversion process. Thus, for example, in some of the processes described, a decrease in the conversion rate of more than 5% after 5 days or of about 1.5-1.8% after 10.5 days, respectively, was reported.